📚 Microorganisms in the Nitrogen Cycle | 氮循环中的微生物
The nitrogen cycle is a vital biogeochemical process that ensures the continuous supply of nitrogen, a key component of proteins and DNA, to living organisms. Microorganisms play an indispensable role in transforming nitrogen into forms that plants and animals can use, as well as returning it to the atmosphere.
氮循环是一个重要的生物地球化学过程，确保了生物体对氮这一蛋白质与DNA关键成分的持续供应。微生物在将氮转化为植物和动物可利用的形式以及将其送回大气方面扮演着不可或缺的角色。
1. Why Is the Nitrogen Cycle Important? | 氮循环为何重要？
All living organisms require nitrogen to synthesise proteins, nucleic acids (DNA and RNA) and other essential biomolecules. Although the atmosphere is about 78% nitrogen gas (N2), most organisms cannot use it directly. The nitrogen cycle converts inert atmospheric nitrogen into biologically available forms, sustaining ecosystems and food production.
所有生物都需要氮来合成蛋白质、核酸（DNA 和 RNA）以及其他重要的生物分子。尽管大气中约78%是氮气（N2），大多数生物无法直接利用它。氮循环将惰性的大气氮转化为生物可利用的形式，维持了生态系统和粮食生产。
2. Key Forms of Nitrogen in the Cycle | 氮循环中氮的主要形式
In the cycle, nitrogen appears in several chemical forms: atmospheric nitrogen gas (N2), ammonia (NH3) which becomes ammonium ions (NH4+) in water, nitrite ions (NO2-), nitrate ions (NO3-), and organic nitrogen locked inside proteins and nucleic acids of living or dead organisms.
在循环中，氮以几种化学形式出现：大气氮气（N2）、在水中会形成铵离子（NH4+）的氨（NH3）、亚硝酸根离子（NO2-）、硝酸根离子（NO3-），以及锁在活体或死有机体的蛋白质和核酸中的有机氮。
3. Microbial Processes at a Glance | 微生物过程概览
Microorganisms drive four key transformations: decomposition, nitrogen fixation, nitrification and denitrification. Each process involves specific groups of bacteria or fungi that obtain energy and nutrients while altering the oxidation state of nitrogen.
微生物驱动四个关键的转化：分解、固氮、硝化和反硝化。每一个过程都涉及特定的细菌或真菌类群，它们在获取能量与营养的同时改变了氮的氧化状态。
- Decomposition – breaks down organic nitrogen into ammonium (NH4+). | 分解 – 将有机氮分解为铵（NH4+）。
- Nitrogen fixation – converts N2 gas into ammonia (NH3) / ammonium. | 固氮 – 将N2气体转化为氨（NH3）/ 铵。
- Nitrification – oxidises ammonium to nitrite and then to nitrate. | 硝化 – 将铵氧化为亚硝酸盐，再进一步氧化为硝酸盐。
- Denitrification – converts nitrate back into N2 gas. | 反硝化 – 将硝酸盐还原为N2气体。
4. Decomposition: Releasing Ammonium from Organic Matter | 分解：从有机物中释放铵
When plants and animals die or excrete waste, decomposer microorganisms such as bacteria and saprophytic fungi break down the proteins and nucleic acids present in the organic material. Enzymes secreted by these microbes hydrolyse the large molecules, releasing ammonium ions (NH4+) into the soil.
当植物和动物死亡或排泄废物时，分解者微生物（如细菌和腐生真菌）会分解这些有机物质中的蛋白质和核酸。这些微生物分泌的酶水解大分子，将铵离子（NH4+）释放到土壤中。
This process is sometimes called ammonification and makes nitrogen available to plants in a form they can later take up, but only after further oxidation. Without decomposers, dead matter would accumulate and nitrogen would remain locked in unavailable forms.
这一过程有时被称为氨化作用，它使氮以植物随后能够吸收的形式存在，但还需要经过进一步氧化。如果没有分解者，死亡有机质将不断堆积，而氮会一直被锁在不可利用的形式中。
5. Nitrogen Fixation: Converting N2 Gas into Ammonia | 固氮：将N2气体转化为氨
Atmospheric nitrogen (N2) is extremely stable because of the triple covalent bond between the two nitrogen atoms. Nitrogen-fixing bacteria possess the enzyme nitrogenase, which breaks this bond and reduces N2 to ammonia (NH3). In soil solution, ammonia reacts with water to form ammonium ions (NH4+).
大气中的氮气（N2）由于两个氮原子之间的三键共价键而极其稳定。固氮细菌拥有固氮酶，可以断开这个三键并将N2还原为氨（NH3）。在土壤溶液中，氨与水反应生成铵离子（NH4+）。
Fixation occurs in two main ways: free-living soil bacteria (e.g., Azotobacter) fix nitrogen independently, while symbiotic bacteria (e.g., Rhizobium) live inside root nodules of leguminous plants such as peas, beans and clover. The plant provides carbohydrates and the bacteria supply ammonium.
固氮主要有两种方式：土壤中自由生活的细菌（如固氮菌 Azotobacter）独立固氮，而共生细菌（如根瘤菌 Rhizobium）生活在豆科植物（如豌豆、菜豆和三叶草）的根瘤中。植物提供碳水化合物，细菌提供铵。
N2 + 8H+ + 8e- + 16ATP → 2NH3 + H2 + 16ADP + 16Pi
6. Nitrification: Two-Step Oxidation of Ammonium to Nitrate | 硝化：铵被两步氧化成硝酸盐
Nitrification is an aerobic process carried out by two groups of chemoautotrophic nitrifying bacteria. First, bacteria such as Nitrosomonas oxidise ammonium ions (NH4+) to nitrite ions (NO2-). Second, bacteria like Nitrobacter oxidise nitrite to nitrate ions (NO3-). Both steps require oxygen and release energy that the bacteria use.
硝化是一个需氧过程，由两组化能自养的硝化细菌进行。首先，像亚硝化单胞菌（Nitrosomonas）这样的细菌将铵离子（NH4+）氧化为亚硝酸根离子（NO2-）。其次，像硝化杆菌（Nitrobacter）这样的细菌将亚硝酸根氧化为硝酸根离子（NO3-）。这两步都需要氧气，并释放供细菌利用的能量。
NH4+ → NO2- → NO3-
Nitrate is the form most readily absorbed by plant roots through active transport. It can then be assimilated into amino acids and proteins. Therefore, nitrification is crucial for maintaining soil fertility.
硝酸盐是植物根系通过主动运输最容易吸收的形式，随后可被同化为氨基酸和蛋白质。因此，硝化作用对于维持土壤肥力至关重要。
7. Denitrification: Returning Nitrogen to the Atmosphere | 反硝化：将氮送回大气
Denitrifying bacteria, such as Pseudomonas, perform anaerobic respiration using nitrate (NO3-) as a terminal electron acceptor instead of oxygen. They reduce nitrate through a series of intermediates back to nitrogen gas (N2), which is released into the atmosphere. This process occurs mainly in waterlogged, compacted or oxygen-poor soils.
反硝化细菌（如假单胞菌 Pseudomonas）进行无氧呼吸，利用硝酸根（NO3-）代替氧气作为最终的电子受体。它们通过一系列中间产物将硝酸盐还原回氮气（N2），并释放到大气中。这一过程主要发生在积水、紧实或缺氧的土壤中。
While denitrification completes the nitrogen cycle, it reduces the amount of nitrate available for plants and can decrease crop yields if not managed. In natural ecosystems it balances fixation, preventing excessive accumulation of fixed nitrogen.
尽管反硝化作用使氮循环形成闭环，但它减少了可供植物利用的硝酸盐量，管理不当会降低作物产量。在自然生态系统中，它与固氮相平衡，防止固定氮的过度积累。
8. The Whole Cycle: Putting It Together | 整个循环：综合理解
In a typical terrestrial ecosystem, nitrogen flows through the following loop: atmospheric N2 is fixed by bacteria into ammonium; decomposers also release ammonium from organic matter; nitrifying bacteria oxidise ammonium to nitrate; plants absorb nitrate and incorporate it into biomolecules; consumers obtain nitrogen by eating plants or other animals; when organisms die, decomposers return ammonium to the soil; denitrification eventually sends some nitrogen back to the air.
在典型的陆地生态系统中，氮的流动循环如下：大气中的N2被细菌固定为铵；分解者也从有机物中释放铵；硝化细菌将铵氧化为硝酸盐；植物吸收硝酸盐并将其合成为生物分子；消费者通过取食植物或其他动物获得氮；生物死亡后，分解者将铵归还土壤；反硝化作用最终将一部分氮送回空气中。
This cycle is driven entirely by microorganisms, without which nitrogen would become a limiting factor for plant growth and thus for all life.
这个循环完全由微生物驱动，没有它们，氮就会成为植物生长乃至所有生命的限制因子。
9. Nitrogen Cycle Summary Table | 氮循环总结表格
| Process | 过程 | Microorganisms involved | 涉及的微生物 | Starting form | 起始形式 | End form | 最终形式 | Oxygen requirement | 氧需求 |
| Nitrogen fixation | 固氮 | Rhizobium, Azotobacter | N2 | NH3 / NH4+ | Aerobic or microaerobic |
| Decomposition / Ammonification | 分解/氨化 | Bacteria, fungi (saprophytes) | Organic nitrogen | NH4+ | Aerobic / Anaerobic |
| Nitrification | 硝化 | Nitrosomonas, Nitrobacter | NH4+ | NO2- → NO3- | Strictly aerobic |
| Denitrification | 反硝化 | Pseudomonas | NO3- | N2 | Anaerobic |
10. Agricultural and Environmental Link | 农业与环境的联系
Understanding the nitrogen cycle helps farmers improve soil fertility. Planting leguminous crops enriches the soil with nitrogen-fixing bacteria, reducing the need for synthetic fertilisers. Adding manure or compost increases organic matter, boosting ammonification and nitrification.
理解氮循环有助于农民改善土壤肥力。种植豆科作物能够丰富土壤中的固氮细菌，减少对合成肥料的需求。施用粪肥或堆肥增加了有机质，促进了氨化作用和硝化作用。
However, overuse of nitrogen fertilisers can lead to nitrate leaching into water bodies, causing eutrophication – a rapid growth of algae that depletes oxygen and harms aquatic life. Waterlogged fields also promote denitrification, wasting valuable nitrate and releasing N2O, a potent greenhouse gas.
然而，过量使用氮肥会导致硝酸盐淋溶进入水体，引发富营养化——藻类疯长消耗水中氧气，危害水生生物。积水农田还会促进反硝化作用，浪费宝贵的硝酸盐并释放强效温室气体 N2O。
11. Common Misconceptions and Exam Tips | 常见误解与考试技巧
One of the most common mistakes is to think that plants absorb nitrogen gas (N2) directly from the air. In reality, plants take up nitrogen mainly as nitrate ions (NO3-) from the soil through their roots. Another confusion is mixing up nitrification and denitrification – remember that nitrification adds oxygen and occurs in well-aerated soils, while denitrification removes oxygen from nitrate and takes place in anaerobic conditions.
最常见的错误之一是认为植物直接从空气中吸收氮气（N2）。实际上，植物主要通过根部从土壤中吸收硝酸根离子（NO3-）。另一个易混淆点是将硝化和反硝化搞混——记住，硝化加氧并发生在通气良好的土壤中，而反硝化从硝酸盐中夺走氧，发生在厌氧条件下。
When answering Edexcel IGCSE questions, always use precise terminology: say 'nitrogen-fixing bacteria' not just 'bacteria', and state the chemical name (ammonium, nitrite, nitrate) rather than vague phrases like 'nitrogen compounds'. Clearly link the process to the correct microorganism group and describe the direction of conversion.
在回答 Edexcel IGCSE 问题时，要使用精确的术语：说“固氮细菌”而不只是“细菌”，并使用化学名称（铵、亚硝酸盐、硝酸盐），而不是“含氮化合物”等模糊说法。将过程清晰地与正确的微生物类群联系起来，并描述转化方向。
12. Key Vocabulary for Revision | 复习关键词汇
- Nitrogen fixation | 固氮 – conversion of N2 into ammonia/ammonium. | 将 N2 转化为氨/铵。
- Nitrification | 硝化 – two-step oxidation of ammonium to nitrite then nitrate. | 两步氧化，将铵依次转化为亚硝酸盐和硝酸盐。
- Denitrification | 反硝化 – reduction of nitrate to nitrogen gas by anaerobic bacteria. | 厌氧细菌将硝酸盐还原为氮气。
- Ammonification | 氨化作用 – release of ammonium from dead organic matter during decomposition. | 分解过程中从死亡有机质释放铵。
- Root nodules | 根瘤 – structures on legume roots housing symbiotic Rhizobium. | 豆科根部容纳共生根瘤菌的结构。
- Eutrophication | 富营养化 – nutrient enrichment of water causing algal bloom and oxygen depletion. | 水体营养丰富导致藻类爆发和缺氧。
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